Black layer coated heat exchanger

ABSTRACT

A primary heat exchanger of a condensing furnace or a single heat exchanger of a mid-efficiency furnace is electrochemically coated with a coating of copper metal. The copper coating is oxidized with an aqueous oxidizing alkaline solution to form a matte black layer of cupric oxide. As the layer of cupric oxide is black, the layer has a high emissivity and emits more heat, increasing the efficiency of the primary heat exchanger. Alternatively, iron can be electrochemically coated on the primary heat exchanger and oxidized to black magnetite to increase emissivity.

BACKGROUND OF THE INVENTION

[0001] The present invention relates generally to a heat exchanger foruse with the primary heat exchanger of a condensing furnace or thesingle heat exchanger of a mid-efficiency furnace which includes a layerof oxidized black coating which increases the emissivity of the heatexchanger and allows the heat exchanger to emit more heat.

[0002] A condensing furnace generally includes two heat exchangers, aprimary heat exchanger and a condensing heat exchanger. Air and naturalgas enter a burner where they are burned to form hot combustionproducts. The primary heat exchanger cools the hot combustion products,extracting and supplying heat to the air that is to be heated. Astandard (“mid-efficiency”) furnace generally includes only a singleheat exchanger which cools the hot combustion products, extracting andsupplying heat to the air to be heated.

[0003] These heat exchangers are commonly formed of a shiny aluminizedsteel. As the aluminized steel is shiny, the steel does not radiate heatwell and has a low emissivity. Emissivity is the ability of a surface toemit heat by radiation. For example, black bodies have a higheremissivity than lighter bodies and are therefore able to emit more heatby radiation than lighter bodies.

[0004] There are several drawbacks to the aluminized steel heatexchanger of the prior art. For one, aluminized steel is expensive.Additionally, as the aluminized steel has a low emissivity, the steeldoes not radiate heat well.

[0005] Hence, there is a need in the art for a furnace heat exchangerwhich increases the emissivity of the heat exchanger and allows the heatexchanger to emit more heat.

SUMMARY OF THE INVENTION

[0006] The present invention relates to a furnace heat exchanger for usewith the primary heat exchanger of a condensing furnace or the singleheat exchanger of a mid-efficiency furnace which includes a layer ofoxidized black coating which increases the emissivity of the heatexchanger and allows the heat exchanger to emit more heat.

[0007] A furnace heat exchanger made of steel alloy or metal-coatedsteel is electrochemically coated with copper metal. An aqueous solutionof an oxidizing alkaline inorganic compound oxidizes the copper,producing a black matte layer of cupric oxide (CuO).

[0008] As the cupric oxide layer is black, the layer has a highemissivity and emits more heat, increasing the efficiency of the heatexchanger. Also, as cupric oxide has a high decomposition temperature,the cupric oxide is not affected by the flame of the burner which maycontact part of the heat exchanger. It is preferable that cupric oxidebe coated on both sides of the heat exchanger to further increaseefficiency.

[0009] Alternatively, the furnace heat exchanger is formed of a steelalloy and is controllably oxidized to form a black coating of magnetite(Fe₃O₄) which increase the emissivity of the heat exchanger.

[0010] Accordingly, the present invention provides a furnace heatexchanger which includes a layer of oxidized black coating whichincreases the emissivity of the heat exchanger and allows the heatexchanger to emit more heat.

[0011] These and other features of the present invention will be bestunderstood from the following specification and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The various features and advantages of the invention will becomeapparent to those skilled in the art from the following detaileddescription of the currently preferred embodiment. The drawings thataccompany the detailed description can be briefly described as follows:

[0013]FIG. 1A illustrates a schematic diagram of a condensing furnacesystem;

[0014]FIG. 1B illustrates a schematic diagram of a mid-efficiencyfurnace system;

[0015]FIG. 2 illustrates a heat exchanger with a coating of copperelectrochemically applied to the surface of the heat exchanger;

[0016]FIG. 3 illustrates a heat exchanger with a coating of cupricoxide;

[0017]FIG. 4 illustrates a heat exchanger with a coating of cupric oxideon both the inner surface and the outer surface of the heat exchanger;and

[0018]FIG. 5 illustrates a heat exchanger with a coating of cupric oxideon the inner surface and a coating a magnetite on the outer surface ofthe heat exchanger.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0019]FIG. 1A schematically illustrates a condensing furnace system 10A.Air and natural gas enters a burner 12A which burns the air and naturalgas by a flame 11 to produce hot combustion products. The hot combustionproducts pass through a primary heat exchanger 14A, which cools the hotcombustion products and extracts heat to the air to be heated. Toincrease the efficiency of the system 10A, a condensing heat exchanger16 is used to extract additional heat. As the hot combustion gases passthrough the condensing heat exchanger 16A, the condensing heat exchanger16A cools the combustion products to a temperature below the dewpoint ofthe combustion products. Water vapor begins to condense, allowing moreheat to be extracted from the combustion products and increasingefficiency. As the liquid condensate condenses, heat is transferred fromthe water vapor to the air to be heated An inducer fan 18A provides asource of suction on the condensing heat exchanger 16A and assists inpulling the flow of the combustion products through the system 10A. Thecombustion products are expelled from the system 10A through a flue 20A.

[0020]FIG. 1B schematically illustrates a mid-efficiency furnace system10B, which works similarly to the condensing furnace system 10A, butdoes not include a condensing heat exchanger 16A. Heat from the hotcombustion products produced by the burner 12B is extracted by thesingle heat exchanger 14B and supplied to the air to be heated. Theinducer fan 18B provides suction on the combustion products which areexpelled through the flue 20B.

[0021] As shown in FIG. 2, a coating of copper metal 22 is applied tothe surface 24 of the heat exchanger 14 to assure adhesion. The heatexchanger 14 is either a primary heat exchanger 14A of a condensingfurnace system 10A or a single heat exchanger 14B of a mid-efficiencysystem 10B. The heat exchanger 14 is made of either a steel alloy or ametal-coated steel. Preferably, the coating of copper metal 22 iselectrochemically applied to the surface 24. However, it is to beunderstood that other suitable methods of attachment are possible, andone skilled in the art would know who to attach the copper metal 22 tothe surface 24 of the heat exchanger 14. An aqueous solution of anoxidizing alkaline inorganic compound is applied to the copper coating22. An aqueous oxidizing alkaline solution contains an excess ofhydroxide (OH⁻) ions and an oxidizing compound. The hydroxide ions reactwith and oxidize the copper coating 22, producing a layer of cupricoxide (CuO) 26, shown in FIG. 3. The heat exchanger 14 is eitherimmersed in a bath of the aqueous oxidizing alkaline solution, or thesolution is applied as a spray.

[0022] The layer of cupric oxide 26 on the heat exchanger 14 is matteblack. As black bodies emit more heat, the black cupric oxide coatedheat exchanger 14 has a higher emissivity than the shiny aluminizedsteel heat exchanger of the prior art. Emissivity is the relative powerof a surface to emit heat by radiation. As the cupric oxide layer 26 ofthe heat exchanger 14 is matte black, the heat exchanger 14 has a highemissivity and a greater ability to emit heat.

[0023] Additionally, the cupric oxide has a melt or decompositiontemperature of 1326° C. The decomposition temperature is the temperatureat which the cupric oxide is affected or melted by heat. As the heatexchanger 14 does not reach a temperature of over 650° C., the layer ofcupric oxide 26 will not melt or decompose due the heat of the flame 11of the burner 12 which may contact part of the heat exchanger 14.

[0024] As the matte black cupric oxide layer 26 has a high emissivity,the heat exchanger 14 is more efficient. In a furnace where only a heatexchanger 14 is utilized, both the size of the heat exchanger 14 and thefurnace can be reduced, reducing manufacturing costs. Alternatively, ifthe primary heat exchanger 14 remains the same size, the efficiency ofthe heat exchanger 14 is increased as there is a lower gas flue exittemperature. As the efficiency of the heat exchanger 14 is increased,the size of the condensing heat exchanger 16 can be reduced. This isadvantageous as the material of the condensing heat exchanger 16 isexpensive as it must be made resistant to corrosion.

[0025] As shown in FIG. 4, efficiency can further be increased bycoating both the inner surface 28 and the outer surface 30 of the heatexchanger 14 with cupric oxide. By coating both surfaces 28 and 30, theemissivity of the heat exchanger 14 is further increased, allowing foran additional increase in efficiency.

[0026] In an alternative embodiment, a heat exchanger 14 formed of asteel alloy is electrochemically coated with iron. The iron iscontrollably oxidized with an aqueous alkaline solution chemicallyformulated to form a black coating of magnetite (Fe₃O₄). It ispreferable that magnetite only be utilized on the outer surface 30 ofthe heat exchanger 14 as heat from the flame of the heat exchanger 14can oxidize the black magnetite on the inner surface 28 further and turnit red, lowering efficiency. In one embodiment, as shown in FIG. 5, alayer of magnetite 32 is utilized on the outer surface 30 of the heatexchanger 14 which is exposed to the environment and a layer of cupricoxide 26 is used on the inner surface 28 of the heat exchanger 14. Thelayer of magnetite 32 also has a high decomposition temperature and doesnot decompose or melt due to the heat of the condensing furnace system10.

[0027] There are several advantages to using the black coated heatexchanger 14 of the present invention. For one, the aluminized steelheat exchanger of the prior art is shiny so it does not radiate heatwell. As the matte black cupric oxide layer 26 on the heat exchanger 14has a higher emissivity, the heat exchanger 14 can emit more heat andthe efficiency of the heat exchanger 14 is increased. Finally, cupricoxide layer 26 is stable and does not decompose due to the hightemperatures the heat exchanger 14 is exposed to.

[0028] Accordingly, the present invention provides a heat exchanger fora condensing furnace which includes a layer of an oxidized black coatingwhich increases the emissivity of the heat exchanger and allows the heatexchanger to emit more heat.

[0029] The foregoing description is only exemplary of the principles ofthe invention. Many modifications and variations of the presentinvention are possible in light of the above teachings. The preferredembodiments of this invention have been disclosed, however, so that oneof ordinary skill in the art would recognize that certain modificationswould come within the scope of this invention. It is, therefore, to beunderstood that within the scope of the appended claims, the inventionmay be practiced otherwise than as specially described. For that reasonthe following claims should be studied to determine the true scope andcontent of this invention.

What is claimed is:
 1. A method for making a heat exchanger of a furnacesystem comprising the steps of: applying a layer of an oxidizablematerial on a first surface of said heat exchanger; and oxidizing saidlayer of oxidizable material to form a layer of dark material having ahigh emissivity.
 2. The method as recited in claim 1 wherein the step ofapplying said layer of said oxidizable material includes applying saidlayer of oxidizable material electrochemically.
 3. The method as recitedin claim 1 wherein step of oxidizing said layer of oxidizable materialincludes utilizing an oxidizing alkaline solution to oxidize said layerof oxidizable material.
 4. The method as recited in claim 1 wherein saidlayer of oxidizable material is copper.
 5. The method as recited inclaim 4 wherein said layer of oxidizable material is oxidized to cupricoxide.
 6. The method as recited in claim 1 wherein said layer ofoxidizable material is iron.
 7. The method as recited in claim 6 whereinsaid layer of oxidizable material is oxidized to magnetite.
 8. Themethod as recited in claim 1 further including the step of applying saidlayer of oxidizable material to a second surface of said heat exchangercomponent.
 9. The method as recited in claim 1 wherein said firstsurface is an inner surface and said layer of oxidizable materialapplied on said first surface is copper which is oxidized to cupricoxide and said second surface is an outer surface and said layer ofoxidizable material applied on said second surface is iron which isoxidized to magnetite.
 10. A heat exchanger of a furnace systemcomprising: a first surface; and a layer of dark material having a highemissivity applied on said first surface.
 11. The heat exchanger asrecited in claim 10 wherein said layer of dark material is cupric oxide.12. The heat exchanger as recited in claim 10 wherein said layer of darkmaterial is magnetite.
 13. The heat exchanger as recited in claim 10wherein said heat exchanger is made of a steel alloy.
 14. The heatexchanger as recited in claim 10 wherein said heat exchanger is made ofa metal coated steel.
 15. The heat exchanger as recited in claim 10wherein said heat exchanger is a primary heat exchanger.
 16. The heatexchanger as recited in claim 10 wherein heat exchanger further includesan opposing second surface and said layer of dark material is alsoapplied on said opposing second surface.
 17. The heat exchanger asrecited in claim 10 wherein said first surface of said heat exchanger isan inner surface and said layer of material applied on said firstsurface is cupric oxide and said second surface of said heat exchangeris an outer surface and said layer of dark material applied on saidsecond surface is magnetite.
 18. A primary heat exchanger of acondensing furnace system comprising: an inner surface and an outersurface; and a layer of cupric oxide having a high emissivity applied onsaid inner surface and said outer surface.
 19. The heat exchanger asrecited in claim 18 wherein said heat exchanger is made of a steelalloy.
 20. The heat exchanger as recited in claim 18 wherein said heatexchanger is made of a metal coated steel.